While Hunting for a Giant Black Hole, Astronomers Discover a Nest of Darkness

Two astronomers recently searched for a large black hole, as they filtered data from the most powerful earth and space observatories, looking for any evidence of an invisible object hundreds of times the mass of the sun in a cloud far from the stars known as "NGC 6397", but instead they found a nest of small black holes in an area barely larger than our solar system, throwing their considerable weight around in the dense core of the star cluster

The dense core of a globular cluster

Eduardo Vitral and Gary A. Mamon of the Paris Institute of Astrophysics, using data from the Hubble Space Telescope and the European Space Agency’s Gaia spacecraft, reported their results on Feb. 11 in the journal Astronomy and Astrophysics.

"We found very strong evidence of an invisible mass in the dense core of the globular cluster, but we were surprised that this extra mass was not point-like." Eduardo Vitral, a graduate student, said in a press release issued by the Institute of Space Science Observatories.

The surprising result gave astronomers a unique and detailed look at the dynamics prevailing inside one of the crowded places in the near universe.

Their work indicates that such clusters are a possible source of gravitational waves that have been discovered by the LIGO and Virgo Interferometer in recent years, which are clusters that arise from colliding black holes.

But this discovery raises questions about the supposed and confusing process by which small black holes - which are only a few times more massive than the mass of the sun - can merge and grow into giant holes that dominate the centers of galaxies and those distant quasars.

The sizes of galaxies and the masses of holes

Black holes - according to Albert Einstein's theory of general relativity - are objects with a strong gravity to the point where even light can't escape from them, and their existence was questionable in the past, but astronomers now agree that the universe is full of them, and most of them are dead stars that collapsed on themselves and disappeared after burning all of their thermonuclear fuel, with masses a bit more than the mass of our sun.

There are other black holes with masses equivalent to millions or billions of suns in the centers of galaxies, and it seems that there is a relationship between the size of the galaxy and the mass of the black hole in its core, but no one knows why or how these supermassive black holes are formed.

One of the hypotheses indicates that these supermassive black holes arose from smaller black holes of stellar size, and in this case there must be black holes with medium masses floating around them, perhaps in the nuclei of small galaxies or parts of galaxies such as "NGC 6397".

The cluster NGC 6397 is one of about 150 spherical clouds of ancient stars that orbit the Milky Way and perhaps predate its formation. It is 13 billion years old and contains about 250,000 stars, all of them old, small and dim. 

Stars that are brighter and heavier in mass burned out long ago and become black holes or other products of stellar decay; Therefore, the cluster is a potential candidate for harboring an intermediate-mass black hole. In fact, earlier studies had suggested that a black hole of about 600 solar masses anchored the center of NGC 6397

A dark mass equal to 1800 suns

To analyze this idea, Dr. Mamon and his student made high-precision observations of the movements of individual stars in the cluster, based on data obtained from the Hubble and Gaia space observatories. It turns out that the faster the stars move, the greater the force of gravity, and thus there must be more mass to hold it in the cluster.

Altogether, 1905 stars were evaluated from Gaia and 7209 stars from Hubble, and it turned out that the stars were already under the influence of an invisible mass gravity, but instead of revolving around a single dark point the stars were moving in every direction, which indicates whatever dark mass was influencing them was not concentrated, but rather extended, but there was no indication of the existence of a supermassive black hole.

“Our analysis indicated that the orbits of the stars are close to random throughout the globular cluster, rather than systematically circular or very elongated,” Dr. Mamon said in an email.

The movements of these stars provided evidence of a dark mass equal to 1,800 suns spread across an area about a third of a light-year wide. About 40,000 regular and luminous stars share this space, but it is very lightweight.

Dr. Mamon said that according to the models of stellar evolution, about two-thirds of this dark matter is black holes with an average mass of 20 suns, and the rest of the dark matter is the remnants of dead stars such as white dwarves or neutron stars.

The future is unclear

Astronomers acknowledge that what will happen next is unclear, as random merging between black holes may cause them to lose mass in the form of gravitational waves, and these mergers can also push some black holes out of mass, and may lead to gravitational interactions with less massive stars in the cluster. This causes it to lose speed and sink to the center of mass, in a process called "dynamical friction."

Dr. Mamon explained that this may bring the black holes to the center in a few million years, where they merge in the form of a medium-mass black hole," but this time window is not comparable to the 13 billion years of the cluster’s existence.

The fact that there is no medium-mass black hole there now indicates that it may never form. Dr. Mamon said that sinking black holes toward the center may stop, because black holes encounter very few stars to exchange energy with them, and "this would prevent the formation of A medium-mass black hole. "

“Our discovery of this invisible concentration in a core-collapse globular cluster is surely something that makes us wonder about many ideas of intermediate-mass black hole formation!” Said Dr. Vitral.


  • https://www.aanda.org/articles/aa/full_html/2021/02/aa39650-20/aa39650-20.html
  • https://www.nytimes.com/2021/02/26/science/astronomy-black-hole-ngc6397.html